Highly uncommon (4)
I believe the answer is:
rare
'highly uncommon' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rare that I've seen before include "Very scarce like valuable antique" , "like Ben Jonson, Jack Young thought" , "not often spotted" , "Not easily come by like undercooked meat" , "Might a four-leaf clover be so" .)